Recently, mobile terminals (mobile stations) such as cell phones have been increasingly made multimedia-compatible for handling a large amount of still images and short-period moving images, and accordingly, large-capacity high-rate data transmission methods are in demand. Examples of such large-capacity high-rate data transmission methods are the high speed downlink packet access (HSDPA) method, and data transmission methods using high speed physical downlink shared channels (HS-PDSCH) in which only the downlink transmission rate (from a base station to a mobile station) is increased.
In such a data transmission method, to transmit a large amount of data from a communications network to a mobile station via a base station, the data is packetized and transmitted using the HS-PDSCH, and to transmit a large amount of data at once to a plurality of mobile stations, this high speed physical downlink shared channel, referred to as HS-PDSCH, is shared by the mobile stations based on scheduling of a base station scheduler. That is, transmission priorities of the mobile stations are determined in each transmission unit of time, and for example, when 15 HS-PDSCHs are provided as code resources, scheduling is carried out so that 10 HS-PDSCHs are assigned to a mobile station with the first transmission priority, three HS-PDSCHs are assigned to a mobile station with the second transmission priority, and two HS-PDSCHs are assigned to a mobile station with the third transmission priority.
In the scheduling, a method known as adaptive modulation is employed, in which, a modulation method and a coding ratio are determined according to quality of a radio channel between the base station and the assigned target mobile stations, and code resources for each assigned target mobile station are determined. Each mobile station informs the base station of the radio channel quality as a channel quality indicator (CQI). The values of CQI are defined as shown in Tables 7A, 7B, 7C, 7D, and 7E in 3GPP Specification TS25.214V5.4.0.
A CQI value derivation method in the HSDPA method is described in 3GPP Specification TS25.214V5.4.0 6A.2.
Namely, the electric power to be received for all HS-PDSCHs of a mobile station is estimated by adding the logarithm of an offset Γ (measured power offset informed by the higher-level layer) to the received common pilot channel (CPICH) power of the mobile station. Furthermore, the mobile station uses the HS-PDSCH power as a desired signal power, calculates its ratio to interference power, and determines the CQI on the basis of the calculated ratio. In 3GPP Specification TS25.433V5.4.0 8.2.17.2, it is mentioned that Γ is set by an upper-level parameter. Specifically, Γ is set in response to the node B application part (NBAP) message “Radio Link Set Up.”
However, these documents neither regulate the method of assignment of mobile stations by a scheduler, nor clearly regulate the definition of the offset Γ, that is, whether Γ is a power offset of all the HS-PDSCHs with respect to the CPICH transmitted by a base station, or a power offset of each mobile station with respect to the CPICH. Therefore, if assignment of mobile stations by a scheduler is not properly carried out, this may result in improper assignment, in that the total of transmitting powers of HS-PDSCHs of a base station may exceed the permitted power. That is, power obtained by subtracting the power of other individual channels and the power of a common channel from the total transmitting power.
The present invention was made in view of these circumstances, and an object thereof is to propose a method of assignment of mobile stations by a scheduler, and provide a base station that restricts all the transmitting powers of HS-PDSCHs within a predetermined power. Another object is to realize efficient power assignment by setting all the transmitting powers of HS-PDSCHs as a maximum within the predetermined power. Furthermore, still another object is to improve the transmission efficiency by correcting CQIs according to correction of power assignment, to fall within the predetermined power.